Thermal imaging is one of a number of techniques for examining or identifying targets or objects. One such technique is passive thermal imaging.
Passive thermal imaging, in which temperature differences in different portions of a target area are detected using infrared cameras or imaging techniques, for example, as described in Gerald C. Hoist, Common Sense Approach to Thermal Imaging, SPIE—the International Society for Optical Engineering, Bellingham, Wash., 2001, has been used for many years. With the use of advanced detectors such as high resolution focal plane arrays and microbolometers, small differences in temperature may be readily detected. However, if the target does not produce its own thermal signature and has remained at the same location for some time, differences in temperature within the target or between the target and background region may be too small to detect.
Passive thermal imaging is routinely used in a variety of situations, including surveillance, nondestructive testing (NDT), electrical/mechanical inspection, building inspection, detection of buried objects, and process/quality control.
Another technique, termed active thermal imaging, involves using an external heating source to enhance thermal contrasts in a target area. These have generally been done over short ranges for nondestructive evaluation applications. Time Resolved Infrared Radiometry (TRIR), an example of such a technique, is described in L. C. Aaomodt, J. W. Maclachan Spicer, and J. C. Murphy, “Analysis of Characteristic Thermal Transit Times for Time-Resolved Infrared Radiometry Studies of Multilayered Coatings”, J. Appl. Phys. 68, 6087 (1990). In most cases, a laser or flashlamp is used to heat the region of the target close to the surface. Because the optical radiation does not penetrate into most targets, thermal diffusion plays a major role. Although the heating originates from the surface, the time-dependent temperature profiles can give information about defects or objects beneath the surface. Subsurface features or objects are eventually heated through thermal diffusion, but their temperature can be different from that of surrounding material. The characteristic transit time for thermal diffusion to heat a subsurface feature at a depth d is td=d2/αd, where αd, is the thermal diffusivity, and the time for that signature to diffuse back to the surface is 2td. This process can be used to image or detect the subsurface feature.
There have been also several reports of using conventional microwave sources to provide the heating. Most have involved sources at the commercial S-band frequency of 2.45 GHz. An example is crack detection in concrete structures, for example as described in S. A. Telenkov, G. Vargas, J. S. Nelson, and T. E. Milner, “Coherent Thermal Wave Imaging of Subsurface Chromophores in Biological Materials,” Phys. Med. Biol. 47, 657 (2002). Other applications are directed to schemes to detect buried mines and unexploded ordinance using microwave heating, e.g. as described in U.S. Pat. No. 6,343,534, S. M. Khanna et al., issued Feb. 5, 2002. TRIR imaging of subsurface microwave absorbers in dielectrics using an X-band (10 GHz) microwave heating source has also been demonstrated, as described in R. Osiander, J. W. M. Spicer, and J. C. Murphy, “Thermal Imaging of Subsurface Microwave Absorbers in Dielectric Materials,” Thermosense XVI, SPIE Vol. 2245, SPIE—the International Society for Optical Engineering, Bellingham, Wash., 1994, p. 111, and in U.S. Pat. No. 6,183,126. J. C. Murphy et al., issued Feb. 6, 2001.
Although it does not use an artificial active source, there is a related passive method that has been used in the past. This method relies on the differential heating (or cooling) that occurs at sunrise or sunset, so that the sun in effect becomes an active heating source. Material that has sufficient thermal inertia compared to its surroundings may maintain its temperature after sunset for some period of time. This method has been used for military targets, including detection of subsurface mines, e.g. as described in J. R. Simard, “Improved Landmine Detection Capability (ILDC): Systematic Approach to the Detection of Buried Mines using Passive IR Imaging,” in Detection and Remediation Technologies for Mines and Minelike Targets, SPIE Vol. 2765, SPIE—the International Society for Optical Engineering, Bellingham, Wash., 1996, p. 489.
For the current active thermal imaging methods that use conventional long wavelength microwaves as the heating source, extending the range beyond a few meters presents fundamental challenges. For a given range, the heating radiation must provide sufficient intensity to raise the surface temperature in the target area by a detectable amount. Conventional S-band microwave sources operating at 2.45 GHz are not ideal because the radiation cannot be focused with a reasonable antenna size, and radiation at this frequency does not couple well to most materials. Thus, the amount of energy required to produce a detectable thermal signature can become unacceptably large.
The current active thermal imaging methods that use optical sources such as lasers or flashlamps provide surface heating and are well suited for many short range applications in a controlled environment. The use of lasers, e.g. as suggested in U.S. Patent Application No. 20040081221, R. Sandvoss, published Apr. 29, 2004, could be applied for longer range remote sensing applications, but lasers with sufficient intensity to produce a strong thermal signature and power to illuminate a large area generally pose an unacceptable eye safety risk. In addition, the scaling of such systems to high average power is generally less favorable than for microwave or millimeter-wave devices.